FIG. 1 shows a functional reference configuration for conventional interworking between a DECT (digital European cordless telephone) system and a GSM (global system for mobiles) system. FIG. 1 shows that the DECT portable part (PP) 1 communicates with the DECT fixed part (FP) 2 over an air interface 3. The fixed part 2 communicates with a GSM mobile switch centre (MSC) 4 over the standard GSM A-interface 5. The fixed part comprises at least one radio fixed part (RFP) 6, which communicates wirelessly with the portable part, and a cluster control fixed part (CCFP) 7 which handles communications between the fixed part and the mobile switch centre.
In the signalling plane of the A-interface mapping takes place only on the CC/NWK and MM/NWK levels, so the DECT system does not use the lower layers of the A-interface. Documents [12] to [18] listed below provide detailed definitions of how GSM services are provided over the DECT air interface.
Document [14] cited below defines the implementation of bearer services. This takes as its basis that in the DECT/GSM interface the GSM error correcting protocol RLP is terminated to the interworking unit (IWU) of the DECT fixed part, and the DECT air interface (based on the C data profile) provides its own error correction over the DECT air interface by re-transmitting the MAC (medium access control) layer protocol and by the DLC (data link control) layer re-transmitting the LAPU protocol.
This protocol model Illustrated in FIG. 2, where block 8 presents the DECT portable part, block 9 represents the DECT fixed part and block 10 represents the GSM mobile switch centre interworking unit. Thus, in this standard implementation the RLP and LAPU protocols function independently; the RLP protocol operating over the A-interface (11 in FIG. 2) between GSM and DECT and the LAPU protocol operating over the DECT air interface (12 in FIG. 2). The LAPU protocol runs all the way from the CCFP to the PP, so during provision of a data service--in which the system carries signals representing data that has been provided to the system in a digital form by a user of the DECT portable part, or that is to be provided by the system to that user in a digital form--situations where data might be lost in DECT internal handover can be taken care of either by the DECT MAC layer error correction or by the LAPU. However, error correction in the event of external handover is unclear.
Currently only the DECT C-data profile defines the use of V.42bis compression over the U-plane link by using C-plane Call Control (CC) negotiation for the compression parameter negotiation. No such functionality is defined for the DECT A/B profiles. (See documents [20] and [22]).
A significant weakness of the system described above is the relatively low data transmission level it provides over the radio interface: the highest available data rate is 9600 bps (bits per second). However, DECT provides a higher capacity over its air interface and could support higher transmission speeds.
The present invention aims to address this problem.
According to the present invention from a first aspect there is provided a DECT fixed part for providing a communications link between a DECT portable part and a GSM mobile switch centre, the flexed part comprising a compression negotiation means for: receiving from one of the portable part and the mobile switch centre a first message specifying a form of compression for a data service; and using that first message, determining a second message to be sent to the other of the portable part and the mobile switch centre to request it to adopt a corresponding form of compression for the data service; so as to allow the provision of a compressed data service between the portable part and the mobile switch centre.
The compression negotiation means preferably acts so as to, in effect, translate messages specifying forms of data service compression between the protocols of the links between the fixed part and the portable part and the fixed part and the mobile switch centre. The first message may suitably be a CC-SETUP message received from the portable part, in which case the second message may suitably be a SETUP message that is sent to the mobile switch centre. The first message may be a SETUP message received from the mobile switch centre, in which case the second message may suitably be a CC-SETUP message that is sent to the portable part.
One or more XID messages between the fixed part and the mobile switch centre may be used to establish an acceptable form of compression.